In general, such works as excavation are executed by wheel loaders in which an operating lever and etc. are operated while a fuel control lever is set in a maximum rotation position. In such a work condition, vibrations during travelling and unavoidable vibrations in such work as excavation are generated, and accordingly, vibrations attributed to engine rotation are supposed to be smaller relatively thereto. Therefore, vibrations attributed to engine rotation do not make an operator feel uncomfortable.
On the other hand, works are often executed while travelling is stopped and simultaneously the engine rotation speed is set to be the maximum rotation speed, for instance, when earth, sand and etc. are excavated and then loaded on a dump truck or the like by operating a boom. Therefore, vibrations attributed to engine rotation are increased. Further, there is herein no external factor related to vibrations. Accordingly, vibrations attributed to engine rotation make an operator feel uncomfortable in such condition.
Now, an engine control device as described in Japan Laid-open Patent Application Publication No. JP-A-2000-303872 has been proposed for inhibiting vibrations in a construction machine. In the device, the engine rotation speed is firstly set to be a first lower rotation speed higher than a rotation speed at a maximum resonance point when an operation is not being executed for a work. Then, after elapse of a predetermined period of time, the engine rotation speed is set to be a second lower rotation speed that is further lower than the first rotation speed and is close to a rotation speed at an anti-resonance point. Thus, through the control of low idle rotation speed, resonance of the engine is avoided, and low fuel consumption and low noise generation are achieved.
On the other hand, a control device described in Japan Laid-open Patent Application Publication No. JP-A-H09-32596 is configured to automatically change the engine rotation speed into a rotation speed higher or lower than a preliminarily set specific rotation speed range when it is determined that the engine rotation speed falls in the specific rotation speed range. Accordingly, the engine is prevented from being operated in the specific rotation speed range with large noise and large vibrations, and dwelling comfort and durability of a vehicle are enhanced.